1. Field
The present embodiments relate to an electrolyte for a lithium secondary battery capable of improving safety and reliability of the lithium secondary battery and a lithium secondary battery comprising the electrolyte.
2. Description of the Related Technology
A mixed solution prepared by dissolving one or more lithium salts selected from LiPF6, LiBF4, LiBOB, LiFOB, LiTFSI, LiBETI and others into a mixed solvent consisting of cyclic esters such as EC, PC, BC, GBL, GVL and others, and chain esters such as DMC, EMC, DEC, DPC, acetate, propionate, butyrate, hexanoate and others is used as an electrolyte for a lithium secondary battery. However, these kinds of organic solvents may cause combustion, and they are vulnerable to safety. In order to decrease the danger, a way of enhancing flame retardancy by additionally mixing flame retarding additives of fluorine-based compounds or phosphorous compounds such as phosphate, phosphazene and others with an existing electrolyte has been examined.
However, it is difficult to practically apply the phosphate-based material to the electrolyte since phosphate-based material deteriorates ion conductivity and is easily subject to reduction in the negative electrode. Therefore, materials such as fluoroalkyl carbonate, fluoroalkyl ester and fluoroalkyl ether have been examined as alternative materials instead of the phosphate-based material.
Fluoroalkyl carbonate and fluoroalkyl ester among the materials are excellent in flame retardancy. However, fluoroalkyl carbonate and fluoroalkyl ester have high side reaction in the negative electrode and weaken interfacial properties. Therefore, it is difficult to use fluoroalkyl carbonate and fluoroalkyl ester in sufficient amounts for applying flame retardancy to the electrolyte. On the contrary, since fluoroalkyl ether has relatively low reductive cleavage in the negative electrode such that it is capable of improving safety of the batteries without greatly deteriorating performance of batteries. However, although the electrolyte should be used in an excessive amount of 50% by volume or more with respect to the total volume of the electrolyte such that fluoroalkyl ether is used to show flame retardancy, properties of the batteries deteriorate by an increase in the viscosity and a decrease in the ion conductivity in this case. Furthermore, there is a limitation that discharge capability during high rate discharge and capability during the cycle life at normal temperature decrease in that case.